Nearly 25 years ago, Hodgson and Fridovich reported that copper zinc superoxide dismutase mixture was able to catalyze the peroxidation of unsaturated lipids. The oxidant responsible for oxidation of lipid was suggested to be a copper-bound hydroxyl radical. To better understand the mechanism of SOD peroxide catalyzed lipid peroxidation, we have used S-nitrosoglutathione (GSNO) as an antioxidant probe. Addition of peroxide (1 mM) to an incubation mixture containing soybean phosphatidylcholine liposomes (250 microg/ml) and SOD (200 micro g/ml) caused an increase in the formation of conjugate diene which was inhibited by the addition of diethylenetriaminepentaacetic acid (DTPA). Addition of GSNO (10-50 microM) caused a concentration-dependent increase in time-lag for conjugate diene formation indicating that GSNO is acting as a nitric oxide donor in this system. Nitric oxide released from GSNO in the presence of SOD and peroxide was measured by chemiluminescence. We a ttri bute the increase in peroxidation caused by SOD and peroxide to the copper ions released from SOD that subsequently catalyzes the lipid hydroperoxide-dependent oxidation. The present results demonstrate the nitric oxide is an excellent chain-breaking antioxidant. The antioxidant activity of GSNO is attributed to the copper ion dependent release of nitric oxide, which acts as a chain breaking antioxidant. The present results also show that the glutamate groups present in nitrosothiols make them less susceptible to copper-catalyzed decomposition.